Though marriage has ancient roots, until recently love had little to do with it.

"What marriage had in common was that it really was not about the relationship between the man and the woman," said Stephanie Coontz, the author of "Marriage, a History: How Love Conquered Marriage," (Penguin Books, 2006). "It was a way of getting in-laws, of making alliances and expanding the family labor force."

But as family plots of land gave way to market economies and Kings ceded power to democracies, the notion of marriage transformed. Now, most Americans see marriage as a bond between equals that's all about love and companionship. [I Don't: 5 Myths About Marriage]

That changing definition has paved the way for same-sex marriage and the June 26 Supreme Court rulings, which struck down the Defense of Marriage Act (DOMA) and dismissed a case concerning Proposition 8.

From polygamy to same-sex marriage, here are 13 milestones in the history of marriage.

1. Arranged alliances

Marriage is a truly ancient institution that predates recorded history. But early marriage was seen as a strategic alliance between families, with the youngsters often having no say in the matter. In some cultures, parents even married one child to the spirit of a deceased child in order to strengthen familial bonds, Coontz said.

2. Family ties

Keeping alliances within the family was also quite common. In the Bible, the forefathers Isaac and Jacob married cousins and Abraham married his half-sister. Cousin marriages remain common throughout the world, particularly in the Middle East. In fact, Rutgers anthropologist Robin Fox has estimated that the majority of all marriages throughout history were between first and second cousins.

3. Polygamy preferred

Monogamy may seem central to marriage now, but in fact, polygamy was common throughout history. From Jacob, to Kings David and Solomon, Biblical men often had anywhere from two to thousands of wives. (Of course, though polygamy may have been an ideal that high-status men aspired to, for purely mathematical reasons most men likely had at most one wife). In a few cultures, one woman married multiple men, and there have even been some rare instances of group marriages. [Life's Extremes: Monogamy vs. Polygamy]

4. Babies optional

In many early cultures, men could dissolve a marriage or take another wife if a woman was infertile. However, the early Christian church was a trailblazer in arguing that marriage was not contingent on producing offspring.

"The early Christian church held the position that if you can procreate you must not refuse to procreate. But they always took the position that they would annul a marriage if a man could not have sex with his wife, but not if they could not conceive," Coontz told LiveScience.

5. Monogamy established

Monogamy became the guiding principle for Western marriages sometime between the sixth and the ninth centuries, Coontz said.

"There was a protracted battle between the Catholic Church and the old nobility and kings who wanted to say 'I can take a second wife,'" Coontz said.

The Church eventually prevailed, with monogamy becoming central to the notion of marriage by the ninth century.

6. Monogamy lite

Still, monogamous marriage was very different from the modern conception of mutual fidelity. Though marriage was legally or sacramentally recognized between just one man and one woman, until the 19th century, men had wide latitude to engage in extramarital affairs, Coontz said. Any children resulting from those trysts, however, would be illegitimate, with no claim to the man's inheritance.

Marriages in the West were originally contracts between the families of two partners, with the Catholic Church and the state staying out of it. In 1215, the Catholic Church decreed that partners had to publicly post banns, or notices of an impending marriage in a local parish, to cut down on the frequency of invalid marriages (the Church eliminated that requirement in the 1980s). Still, until the 1500s, the Church accepted a couple's word that they had exchanged marriage vows, with no witnesses or corroborating evidence needed.

8. Civil marriage

In the last several hundred years, the state has played a greater role in marriage. For instance, Massachusetts began requiring marriage licenses in 1639, and by the 19th-century marriage licenses were common in the United States.

9. Love matches

By about 250 years ago, the notion of love matches gained traction, Coontz said, meaning marriage was based on love and possibly sexual desire. But mutual attraction in marriage wasn't important until about a century ago. In fact, in Victorian England, many held that women didn't have strong sexual urges at all, Coontz said.

10. Market economics

Around the world, family-arranged alliances have gradually given way to love matches, and a transition from an agricultural to a market economy plays a big role in that transition, Coontz said.

Parents historically controlled access to inheritance of agricultural land. But with the spread of a market economy, "it's less important for people to have permission of their parents to wait to give them an inheritance or to work on their parents' land," Coontz said. "So it's more possible for young people to say, 'heck, I'm going to marry who I want.'"

Modern markets also allow women to play a greater economic role, which lead to their greater independence. And the expansion of democracy, with its emphasis on liberty and individual choice, may also have stacked the deck for love matches.

11. Different spheres

Still, marriage wasn't about equality until about 50 years ago. At that time, women and men had unique rights and responsibilities within marriage. For instance, in the United States, marital rape was legal in many states until the 1970s, and women often could not open credit cards in their own names, Coontz said. Women were entitled to support from their husbands, but didn't have the right to decide on the distribution of community property. And if a wife was injured or killed, a man could sue the responsible party for depriving him of "services around the home," whereas women didn't have the same option, Coontz said.

12. Partnership of equals

By about 50 years ago, the notion that men and women had identical obligations within marriage began to take root. Instead of being about unique, gender-based roles, most partners conceived of their unions in terms of flexible divisions of labor, companionship, and mutual sexual attraction.

"One of the reasons for the stunningly rapid increase in acceptance of same sex marriage is because heterosexuals have completely changed their notion of what marriage is between a man and a woman," Coontz said. "We now believe it is based on love, mutual sexual attraction, equality and a flexible division of labor."